My current kernel was generated with genkernel from gentoo-sources-3.5.7 for my first Gentoo installation and works well up to now.
After emerging world i noticed that gentoo-sources-3.8.13 is available, and I've like to upgrade the kernel (for no particular reason but for have a newer and maybe "better" version).
This version is marked as stable, and my 3.5.7 is no more in http://packages.gentoo.org , why? Shall I upgrade the kernel?
A full manual configuration of the kernel is too difficult for me, so I thought genkernel could be helpful.

After the first compilation (--no-menuconfig) my wlan0 wasn't obviously in "ifconfig -a", and "modprobe ath9k" gave me an error.
I recompiled the kernel (--menuconfig) in this way:

This is a good idea (setting it, I mean -- so it shows [*] at the beginning rather than [ ]), as lots of tools still require wext (iwconfig, many wifi monitors, etc.).

If you just want to get a connection, you could also use wpa_supplicant with -Dnl80211 instead of -Dwext to make use of the newer config interface. (Or, perhaps better yet, you could do both of these to make use of the newer interface for wpa_supplicant, but still retain compatibility with older tools. The newer interface has some benefits, like more detailed error reporting to wpa_supplicant, for instance.)